We discuss the functional roles of-adrenergic receptors in skeletal muscle hypertrophy and atrophy as well as the adaptive responses of-adrenergic receptor expression to anabolic and catabolic conditions.-Adrenergic receptor stimulation using anabolic drugs increases muscle mass by promoting muscle protein synthesis and/or attenuating protein degradation. These effects are prevented by the downregulation of the receptor. Endurance training improves oxidative performance partly by increasing-adrenergic receptor density in exercise-recruited slow-twitch muscles. However, excessive stimulation of-adrenergic receptors negates their beneficial effects. Although the preventive effects of-adrenergic receptor stimulation on atrophy induced by muscle disuse and catabolic hormones or drugs are observed, these catabolic conditions decrease-adrenergic receptor expression in slow-twitch muscles. These findings present evidence against the use of-adrenergic agonists in therapy for muscle wasting and weakness. Thus,-adrenergic receptors in the skeletal muscles play an important physiological role in the regulation of protein and energy balance.